RT9912A RICHTEK [Richtek Technology Corporation], RT9912A Datasheet - Page 10

• Manufacturer Part Number: RT9912A
• Description: Multi-Channel Power Management IC for Portable Device
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9912A
Application Information
RT9912A is a four-channel power management IC (PMIC)
including one step-up DC-DC converter (Boost), one step-
down DC-DC converter (Buck), one low dropout regulator
(LDO) and one voltage detector. For optimizing the
application of portable hand-held system with one or two
alkaline battery, several special logics are designed in
this chip. An external P-MOSFET is also needed for load-
disconnected function.
Step-Up DC-DC Converter (Boost)
The step-up DC-DC convert can start up even with the
input voltage as low as 0.8V and operates with the input
voltage down to 0.7V. The cost of system is reduced by
the internal synchronous rectifier from eliminating an
external Schottky diode. The efficiency of light load is
improved by the pulse frequency modulation mode (PFM)
low quiescent current 30uA. The efficiency of heavy load
is also maintained by the internal synchronous rectifier
with resistance low to 0.2Ω.
The step-up DC-DC converter is designed as a
bootstrapped structure. As the chip is in the start-up period,
a low voltage start-up circuit will pull the output voltage to
a higher voltage (~1.5V). After the output voltage reaches
a certain level, the main DC-DC circuitry will keep working
to pull the output voltage to the expected value set by
output divided resistor. The control scheme of the step-
up DC-DC converter is pulse frequency modulation mode
(PFM) with constant-on-time and minimum-off-time. This
scheme can keep high efficiency during a wide load range.
An internal soft-start is also included in the step-up DC-
DC converter to limit the inrush current to less than a half
of the OCP level. As the ENBST is pulled low, the step-up
DC-DC converter will enter shutdown mode and all function
will be disabled.
As the ENSW is pulled high, the PSW will be pulled low
when the output of the step-up DC-DC converter is ready
(soft-start is finished). An external P-MOSFET is needed
to be a load-disconnected switch. The PSW is a signal
to control the external P-MOSFET. All loadings of the
system should be connected to the drain pin of the
P-MOSFET to prevent the step-up DC-DC converter start-
up in heavy load condition.
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10
The maximum duty (D) of the step-up DC-DC converter is
around 50% so that the maximum output voltage is ideally
to be V
drop on the internal N-MOSFET and inductor. Therefore,
the maximum output voltage will be lower than the ideal
value and to be 2 x V
The function of R9 and C14 is preventing the charge sharing
issue from the capacitor in Q1's source pin to the capacitor
in Q1's drain pin. If the capacitor in Q1's source pin is 10
times larger than the capacitor in Q1's drain pin, R9 and
C14 can be removed.
Step-Down DC-DC Converter (Buck)
The step-down DC-DC convert can reduce the cost of
system by the internal synchronous rectifier from
eliminating an external Schottky diode. The light load
efficiency is improved by the pulse frequency modulation
mode (PFM) and internal synchronous rectifier. For heavy
load, the efficiency is maintained by the internal
synchronous rectifier with the resistance low to 0.4Ω. The
control scheme of the step-down DC-DC converter is pulse
frequency modulation mode (PFM) with over-current-
protection (OCP) and minimum-off-time. This scheme can
keep high efficiency during a wide load range. An internal
soft-start is also included in the step-down DC-DC
converter to limit the inrush current less than a half of
OCP level.
This step-down converter can operate in low-drop mode
and its output voltage depends on the voltage drop cross
the internal P-MOSFET and inductor. Normally, the value
is near VDD2 as the ESR of inductor is 0.1Ω and 60mA
loading. The minimum output voltage is 0.6V, which is
decided by the operation range of the internal circuit.
Low Dropout Regulator (LDO)
The low dropout regulator can regulate the output voltage
by setting the external resistor of FB3. An internal
compensation structure is designed for keeping stability
as wide range output capacitor and wide range loading.
The voltage detector is a comparator with reference to
detect the voltage of FB4.
IN
/ (1-D) = 2 x V
IN
.
IN
. Actually, some voltage will
DS9912A-01 April 2011